1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
2. Related Art
Semiconductor devices have elicited high performance through device miniaturization. As for insulating films, materials (high-permittivity (high-k) materials) with higher relative permittivity than the relative permittivity of conventional silicon oxide films are being used so as to further reduce the film thickness.
For example, in a nonvolatile semiconductor memory device that can perform electric writing and erasing (a EEPROM), the use of a high-permittivity insulator for the blocking layer (the interelectrode insulating film) formed between the floating gate electrode and the control gate electrode is being considered, so as to reduce the device size and increase the coupling ratio between the floating gate electrode and the control gate electrode. One example of such a high-permittivity insulator is LaAlOx, which has high permittivity
∈(approximately 23), and has a wide band gap while maintaining the high permittivity. Having high stability and not easily forming a low-permittivity layer, LaAlOx is also considered to be suitable for the film to be formed on a Si film.
Recently, however, there have been reports of the problem that mutual diffusion is caused between the LaAlOx layer and the polycrystalline Si layer used as the floating gate electrode in a stack structure, where high-temperature heat treatment is carried out (see FIG. 1 in “Field-effect transistors with LaAlO3 and LaAlOxNy gate dielectrics deposited by laser molecular-beam epitaxy” by X. B. Lu et al., Applied Physics Letters, vol. 85, No. 16, p.3543, 2004, for example). Therefore, in a case where LaAlO3 is used as the interelectrode insulating film, the film thickness of the interelectrode insulating film becomes larger. When heat treatment is carried out at 950° C. for 30 seconds, for example, 100 nm of Al is diffused. The influence of such diffusion is significant in miniaturization of the floating gate electrodes of future nonvolatile semiconductor memory devices. Also, the interface between the floating gate electrode and the blocking layer (the interelectrode insulating film) becomes unclear due to such mutual diffusion, and the pressure resistance of the device is lowered. As a result, the device characteristics such as write, erase, and charge retention characteristics of the nonvolatile semiconductor memory device might be degraded.
Further, as nonvolatile semiconductor memory devices have become smaller, the use of a metal gate as the control gate electrode is being considered in recent years, so as to reduce parasitic resistance and restrain electrode depletion. Also, the use of a FUSI (Fully Silicided) gate is being considered, as a FUSI gate has high compatibility with the current manufacturing process. However, being a silicide, a FUSI electrode contains Si therein, and might cause mutual diffusion between the floating gate electrode and the FUSI electrode. In a field effect transistor having metal/insulator/semiconductor junctions (a MISFET), on the other hand, the use of a high-permittivity insulating film as the gate insulating film is being considered, so as to secure a sufficient gate capacity without a reduction in physical thickness of the insulating film. When the high-permittivity film is formed or when heat treatment is carried out after the film formation, the interface is oxidized by the oxygen desorbed from the high-permittivity insulating film, and also causes interface states or fixed charges at the same time.
To counter this problem, the interfacial characteristics are improved by inserting a thin silicon oxide film in the interface between a silicon substrate and a high-permittivity gate insulating film. However, the insertion of a silicon oxide film with low permittivity makes the film thinning difficult.
Also, there have been reports of the problem that a high-permittivity material crystallizes during high-temperature heat treatment and degrades the characteristics of the insulating film. To counter this problem, nitrogen introduction into such a high-permittivity material has been suggested to restrain the crystallization. However, nitrogen existing in the interface between a high-permittivity insulating film and a silicon substrate greatly degrades the interfacial characteristics. Particularly, in a case where a p-MOSFET is in an ON state, the threshold value shifts in the negative direction, and the driving current becomes lower (NBTI (Negative Bias Temperature Instability)). This is undesirable in view of long-term reliability. In a case where a LaAlO3 layer as a high-permittivity material is formed on a Si substrate, mutual diffusion is caused between the Si substrate and the LaAlO3 layer when heat treatment is carried out at 950° C. for 30 seconds (see FIG. 2 in “Outdiffusion of La and Al from amorphous LaAlO3 in direct contact with Si(001)” by P. Sivasubramani et al., Applied Physics Letters 86, 201901 (2005), for example). Due to the mutual diffusion, a silicate with low permittivity is formed between the LaAlO3 layer and the Si substrate.
It is known that, if heat treatment is carried out at 1000 ° C. for 60 seconds in a structure having a LaAlO3 layer and a polycrystalline Si layer stacked on a Si substrate, a reaction is caused in the interface between the polycrystalline Si layer and the LaAlO3 layer earlier than in the interface between the Si substrate and the LaAlO3 layer (see FIG. 1 in “Field-effect transistors with LaAlO3 and LaAlOxNy gate dielectrics deposited by laser molecular-beam epitaxy” by X. B. Lu et al., Applied Physics Letters, vol. 85, No. 16, p.3543, 2004, for example).
As described above, with a stack structure formed with a polycrystalline silicon layer or a silicon substrate and a high-permittivity layer, there is the problem of mutual diffusion caused between the polycrystalline silicon layer or the silicon substrate and the high-permittivity layer.